The most common reason for Urchin to report a higher number of Netscape browsers in the Browsers report is that a significant number of Web spiders and robots report themselves as "Mozilla/3.0 (compatible)" or something similiar. Based on this string, Urchin will report hits from these spiders/robots as "Netscape 3.0". Another clue is that this is happening is that the Top Combos report will show a corresponding large number of "Netscape 3.0 - (unknown)" entries. Since no platform information is reported, Urchin puts these hits in the (unknown) category.
Most well-behaved spiders/robots will correctly report themselves as the actual entity they are - e.g. Googlebot, FAST WebCrawaler, Scooter, etc. You may want to have a look at the Web Robots Page for further information on robots/spiders and how best to handle them. In addition, you can use the filtering capabilities of Urchin to strip out unwanted hits from spiders and robots.